In addition to recognizing antigens (Ags) via the T cell receptor (TCR), CD4+T cells can recognize microbial patterns through pattern recognition receptors (PRRs). Human CD4+ T cells can directly recognize mycobacterial lipoproteins via Toll-like receptor 2 (TLR2). In combination with TCR triggering, TLR2 engagement induces CD4+T cell proliferation and secretion of IL-2 and IFN- . The role of TLR2 expressed on CD4+ T cells in T cell activation/differentiation in vivo and its impact on immune responses to M. tuberculosis (Mtb) remains unexplored. One major challenge in understanding T cell TLR2 role in Mtb infection is the lack of adequate animal models to isolate the effects of TLR2 signaling in lymphoid cells from those triggered by TLR2 signaling in myeloid cells. Considering that TLR2 ligands are major components of the Mtb cell wall and traffic inside and outside the infected macrophages, it is important to establish their effects in different immune cells in vivo. This proposal is aimed at defining the role of TLR2 ligand recognition by CD4+ T cells in Mtb infection and after immunization with Mtb antigens using novel in vivo models. We hypothesize that direct recognition of mycobacterial TLR2 ligands by CD4+T cells: a) generates signals that up-regulate Th1 and multifunctional CD4+T cell responses in Mtb infection in vivo; b) results in enhanced immune protection against Mtb; c) can be targeted by vaccine adjuvants to generate stronger memory and recall responses to Mtb. These hypotheses will be tested with two specific Aims: Aim 1. Establish the role TLR2 expressed on CD4+T cells in the immune response to Mtb infection. Aim 2. Determine the impact of mycobacterial TLR2 ligand recognition by CD4+T cells in generation of memory and recall responses to Mtb antigens. To test our hypotheses, we will correlate TLR2 expression on CD4+ T cells with T cell expansion/effector function/differentiation in vivo in an Mtb aerosol infection model. To asses the effect of TLR2 deficiency in CD4+T cells on immune responses to Mtb infection and to immunization we will use two approaches: a) adoptive transfer of Mtb specific-TCR transgenic (Tg) CD4+T cells isolated from Tlr2+/- or from Tlr2-/- mice; b) generation of new conditional knockout mouse strain where TLR2 is selectively deleted in T cells (Tlr2flox/flox x CD4-Cre mice= Tlr2T mice). Our long-term goal is to understand mechanisms that protect against Mtb infection and/or prevent progression to TB disease and apply this knowledge to development of new vaccines. Our immediate goal is to establish the role of TLR2 ligand recognition by CD4+T cells in immune defense against Mtb. Also, we will determine the potential of targeting CD4+T cell expressed-TLR2 with vaccine adjuvants to improve recall responses and protection. In addition to expanding our understanding of the host-pathogen interaction in Mtb infection, this research project may provide new targets for adjuvants, i.e. CD4+T cell TLR2.